1. Field of the Invention
The present invention relates to an optical axis monitoring device and, more particularly, to an optical axis monitoring device for monitoring optical axis variations of light beams in an optical device.
2. Background Technology
The prior art is evidenced by Japanese Utility Model Application No. 57795/83 (Japanese Utility Model Laid Open No. 163943/84) which discloses a device in which a light-receiving element for an optical axis monitoring system is provided in the path of a light beam from a light source. Information on the optical axis variation is obtained from the output of a light-receiving element.
FIG. 4 is a schematic diagram of such a prior art device applied to an optical system of an optical information recording apparatus. Laser beams 2 from a laser beam source 1 are reflected by a mirror 11 and are made to be incident upon a first beam splitter 3-1. The beams are split into a main beam 2a and a sub-beam 2b-1 by the beam splitter 3-1. The sub-beam 2b-1 falls incident on the light-receiving surface of a first light-receiving element 4-1. The main beam 2a is modulated by an electrical signal (not shown) which contains information to be recorded in an optoelectronic modulator 12 and falls incident on a second beam splitter 3-2. In the beam splitter 3-2 the beam is split again into a main beam 2a and a sub-beam 2b-2, and the second sub-beam 2b-2 falls incident on a second light-receiving element 4-2.
The main beam 2a falls incident upon a polarizing beam splitter 14 via a diverging lens 13. The main beam which falls incident upon the polarizing beam splitter 14 propagates straight through the beam splitter 14 to a quarter wavelength plate 15. The beam which passes through the quarter wavelength plate 15 is projected by an objective lens 16 to the recording surface of a recording disk 20.
The light reflected from the recording surface of the recording disk 20 propagates back through objective lens 16 and quarter wavelength plate 15 and falls incident on the polarizing beam splitter 14. The light reflected by the recording surface of the recording disk 20 is redirected at an angle of 90.degree. by the polarizing beam splitter and falls incident on a light-receiving element 18 via a cylindrical lens 17. The amount of displacement of the objective lens 16 in a direction perpendicular to the disk 20 is controlled by the output of the light-receiving element 18. Thus, a focus servo loop is provided to ensure that the recording beams are always focused on the recording surface of the recording disk 20 to obviate misfocusing due to surface warping of the disk 20 or the like.
In an apparatus as set forth in the above in reference to FIG. 4, when a variation in the optical axis of the laser beam source 1 occurs or the laser beam source 1 is replaced, for example, it is possible to bring the main beam 2a into alignment with the normal optical axis by means of the respective outputs of the two light receiving elements 4-1 and 4-2. In such a case, adjustment of the optical axis can be accomplished by a mere adjustment of the mirror 11 or the light source 1 without requiring adjustment of each component of the optical device 30.
The beam splitters 3-1 and 3-2, mentioned above, consists of a polarizing beam splitter or a prism obtained by bonding half prisms together. When a beam splitter of such a construction is used in a signal recording system, the power of light beams that pass through it reaches as high as about 150 mW. Under such circumstances, damage to the polarizing beam splitter is possible due to degradation of the adhesive applied on the contact faces of the two half prisms when they are bonded together.
Accordingly, it is desirable to form the beam splitter with a flat half mirror to split light beams into two parts, and supply one of the two to the monitoring light-receiving element. However, light beams that pass through a flat half mirror undergo an aberration and a shift of optical axis which results in the distortion in the form of a light spot formed on the master disk. This distortion of the light spot causes a problem in that the formation of pits on the master disk is affected.